Baculoviruses express genes in a cascade fashion during an early, a late, and a very late phase involving the switch from a host to a viral RNA polymerase. The virus-encoded DNA-directed RNA polymerase is a novel, multi-functional, and yet a simple enzyme composed of only four equimolar subunits that include 5' (capping) and 3' (polyadenylating) activities. In addition, no other factors are required to specifically recognize and bind to late viral promoters. Baculoviruses are the only nuclear-replicating viruses that encode their DNA-directed RNA polymerase and thus, they lend themselves as a simple eukaryotic model system to study these complex multi-subunit enzymes. The largest subunit of the baculovirus RNA polymerase, late expression factor-8 (LEF-8), has an evolutionarily conserved putative catalytic site shared by a number of enzymes across kingdoms. The motif is found in the b or b' subunit of DNA-directed RNA polymerases in eubacteria and archaebacteria, chloroplasts, Drosophila (RNA polymerase II and III), Saccharomyces (RNA polymerase I, II, and III), and humans (RNA polymerase II). Also, this motif is required in functional assays. The LEF-8 polypeptide will be purified from Escherichia coli and its crystal structure derived. Genetic mutational analysis and DNA binding studies will support the biological significance of the LEF-8 structure. The structure of the baculovirus LEF-8 will be instrumental for drawing structural and functional comparisons between LEF-8 and other eukaryotic and prokaryotic RNA polymerase subunits that share similar domains.